The present invention relates generally to die sets for making food serving disposable pressware such as plates, bowls, trays, or the like from container blanks and more particularly to a punch stripper ring knock-out for purposes of separating the finished product from a die set punch half.
Pressed containers, such as pressed paperboard containers including plates, trays, bowls and the like are well known in the art. Typically, such articles are manufactured on an inclined die set having upper and lower halves. Illustrative in this regard is U.S. Pat. No. 5,249,946 to Marx assigned to the assignee of the present invention. Referring to the ""946 patent, a typical product is manufactured by way of feeding a continuous paperboard web into a cyclically operating blanking section. The forming section includes a plurality of reciprocating upper die halves opposing, in facing relationship, a plurality of lower die halves. The upper die halves are mounted for reciprocating movement in a direction that is oblique or inclined with respect to the vertical plane. The blanks, after cutting, are gravity fed to the inclined lower die halves in the forming section.
Forming operations can be somewhat critical in order to produce quality product at the desired rates. In this respect U.S. Pat. No. 4,721,500 to Van Handle et al. is informative. Note also U.S. Pat. No. 4,609,140 to Van Handle et al. The ""140 patent provides a general description of one known forming method as will be appreciated from FIG. 3 thereof. FIG. 3 shows a cross section of the upper die half and lower die half which are utilized to press a flat, circular paperboard blank into the shape of the plate. The construction of the die halves and the equipment on which they are mounted is substantially conventional; for example, as utilized on presses manufactured by the Peerless Manufacturing Company. To facilitate the holding and shaping of the blank, the die halves are segmented in the manner shown. The lower die has a circular base portion and a central circular platform which is mounted to be moveable with respect to the base. The platform is cam operated in a conventional manner and urged toward a normal position such that it""s flat top forming surface is initially above the forming surface of the base. The platform is mounted for sliding movement to the base, with the entire base itself being mounted in a conventional manner on springs. Because the blank is very tightly pressed at the peripheral rim area, moisture in the paperboard which is driven therefrom during pressing and the heated dies cannot readily escape. To allow the release of this moisture, at least one circular groove is provided in the surface of the base which vents to the atmosphere through a passageway. Similarly, the top die half is segmented into an outer ring portion, a base portion and a central platform having a flat forming surface. The base portion has curved, symmetrical forming surfaces and the outer ring has curved forming surfaces. The central platform in the outer ring are slidingly mounted to the base and biased by springs to their normal position shown in FIG. 3 in a commercially conventional manner. The top die half is mounted to reciprocate toward and away from the lower die half. In the pressing operation, the blank is first laid upon the flat forming surface, generally underling the bottom wall portion of the plate to be formed, and the forming surface makes first contact with the top of the blank to hold the blank in place as the forming operation begins. Further downward movement of the top die half brings the spring-biased forming surfaces of the outer ring into contact with the edges of the blank to begin to shape the edges of the blank over the underlying surfaces in the areas which will define the overturned rim of the finished plate. However, because the ring is spring-biased the paperboard material in the rim area is not substantially compressed or distorted by the initial shaping since the force applied by the forming surfaces is generally light and limited to the spring force applied to the ring. Eventually, the top die half moves sufficiently far down so that the platform segments and the ring are fully compressed such that the adjacent portions of the forming surfaces are coplanar. In a conventional manner the die halves are heated with electrical resistance heaters and the temperature of the die halves is controlled to a selected level by monitoring the temperature of the dies with thermistors mounted in the dies as close as possible to the forming surfaces.
For paperboard plates stock of conventional thicknesses ie. in the range of from about 0.010 to about 0.040 inches, it is preferred that the spacing between the upper die surface and the lower die surface decline continuously from the nominal paperboard thickness at the center to a lower value at the rim.
The springs upon which the lower die half is mounted are typically constructed such that the full stroke of the upper die results in a force applied between the dies of from about 6000 to 8000 pounds.
The paperboard which is formed into the blanks is conventionally produced by a wet laid paper making process and is typically available in the form of a continuous web on a roll. The paperboard stock is preferred to have a basis weight in the range of from about 100 pounds to about 400 pounds per 3000 square foot ream and a thickness or caliper in the range of from about 0.010 to about 0.040 inches as noted above. Lower basis weights in caliper paperboard is preferred for ease of forming and realizing savings in feedstock costs. Paperboard stock utilized for forming paper plates is typically formed from bleached pulp furnish, and is usually double clay coated on one side. Such paperboard stock commonly has a moisture (water content) varying from about 4.0 to about 8.0 percent by weight.
The effect of the compressive forces at the rim is greatest when the proper moisture conditions are maintained within the paperboard: at least 8% and less than 12% water by weight, and preferably 9.5 to 10.5%. Paperboard in this range has sufficient moisture to deform under pressure, but not such excessive moisture that water vapor interferes with the forming operation or that the paperboard is too weak to withstand the high compressive forces applied. To achieve the desired moisture levels within the paperboard stock as it comes off the roll, the paperboard is treated by spraying or rolling on a moistening solution, primarily water, although other components such as lubricants may be added. The moisture content may be monitored with a hand held capacitive type moisture meter to verify that the desired moisture conditions are being maintained. It is preferred that the plate stock not be formed for at least six hours after moistening to allow the moisture within the paperboard to reach equilibrium.
Because of the intended end use of the paper plates, the paperboard stock is typically coated on one side with a liquid proof layer or layers. In addition, for esthetic reasons, the paper plate stock is often initially printed before being coated. As an example of typical coating material, a first layer of polyvinyl acetate emulsion may be applied over the printed paperboard with a second layer of nitrocellulose lacquer applied over the first layer. The plate stock is moistened on the uncoated side after all of the printing and coating steps have been completed. In a typical forming operation, the web of paperboard stock is fed continuously from a roll through a cutting die to form the circular blanks which are then fed into position between the upper and lower die halves. The dies halves are heated as described above, to aid in the forming process. It has been found that best results are obtained if the upper die half and lower die halfxe2x80x94particularly the surfaces thereofxe2x80x94are maintained at a temperature in the range of from about 250xc2x0 F. to about 320xc2x0 F., and most preferably at about 300xc2x0 F.xc2x110xc2x0 F. These die temperatures have been found to facilitate the plastic deformation of paperboard in the rim areas if the paperboard has the preferred moisture levels. At these preferred die temperatures, the amount of heat applied to the blank is apparently sufficient to liberate the moisture within the blank under the rim and thereby facilitate the deformation of the fibers without overheating the blank and causing blisters from liberation of steam or scorching the blank material. It is apparent that the amount of heat applied to the paperboard will vary with the amount of time that the dies dwell in a position pressing the paperboard together. The preferred die temperatures are based on the usual dwell times encountered for normal production speeds of 40 to 60 pressings a minute, and commensurately higher or lower temperatures in the dies would generally be required for higher or lower production speeds, respectively.
Particular forming dies and processes for making pressed paperboard products are likewise well known. Most typically, dies sets for forming paperboard containers include a male or punch die half and a female die half. Typically, the punch half is reciprocally mounted with respect to its opposing die half and both die halves are segmented. One or more portions of the die halves may be spring-biased if so desired, and the particular geometry of the die will depend upon the product desired. In this regard, there is shown in U.S. Pat. No. 4,832,676 to Johns et al. an apparatus for forming a compartmented paperboard plate. The dies illustrated in the ""676 patent includes spring-biased segments as well as pressure rings on the punch half. The particular apparatus further includes articulated, full area knock-outs.
As will be appreciated by one of skill in the art, the knock-outs are important for separating the finished product from the die halves, particularly during high speed operation. The mechanical features can be further augmented pneumatically as is disclosed in U.S. Pat. No. 4,755,128 to Alexander et al. Other patents of interest include:
U.S. Pat. No. 4,435,143 to Dempsey;
U.S. Pat. No. 5,041,071 to Reasinger et al. and
U.S. Pat. No. 4,778,439 to Alexander.
As will be appreciated from the foregoing patents, mechanical full area punch knock-outs, located in the bottom area of the punch half, commonly are used to aid in the removal of the formed pressware products from the punch by actuating a short distance from the punch contour. The full area punch knock-outs typically separate the product from the punch contour successfully but can create a vacuum between themselves and the formed product resulting in slow and inconsistent release. In the worst cases, the formed product is retained in the die set and it is formed into the following product (double pressed). The double pressed product typically will release from the die set since it has more weight and can more easily break the vacuum. The double press products are scrapped by manufacturing or inadvertently packaged and sold to the consumer. Inconsistent product release from the punch can result in lower forming press productivity, increased product manufacturing cost and reduced useful package count to the customer.
Stated another way, the formed product can remain on or experience slow release from the one piece, full area punch knock-outs due to a vacuum created between its smooth surface contacting the product""s smooth coated surface. The suction force created by the vacuum and knock-out surface area (force=pressure (vacuum)xc3x97area) may exceed the weight of a paperboard product. The product thus cannot release until air enters between the product and the knock-out or an air eject blow-off system slides the product sideways along the knock-out surface. Either way can result in slow and inconsistent product release.
Mechanical articulated punch knock-outs provide utility by more positively pushing the blank into the die set bottom to provide for full height product formation. This is especially useful in deeper products, such as bowls where heavy paperboard gathering (pleating) can prevent full height product formation. Articulated knock-outs can provide an even better seal/vacuum between the product and the punch knock-out, since they can wrap around into sidewall areas resulting in inconsistent product release and requiring other mechanical means (ie. split punch flange forming) and/or air assist to remove the products.
Removing the product with air can also have unexpected and inconsistent results. Air flowing across light weight pressware products can result in a creation of a xe2x80x9cBernoulli Effectxe2x80x9d low pressure lift analogous to that experienced by an airplane wing. The air flowing through the small area between the product and punch can be at high velocity and generate low pressure based on Bernoulli""s Principle of conservation of energy. This low pressure can create an upward force that can hold the product against the punch contour and knock-out. Air timing and pressure is critical to remove the products. More air pressure and flow may not provide better release since higher air flow velocities and lift forces can be generated. Just the right amount of air at the correct timing is required to remove the product from the punch, thus resulting in inconsistent results.
The formed pressware products tend to remain on the punch of the die set due to vacuum created between the punch contour, punch knock-out and coated paperboard products particularly. Sudden movement of the punch upwardly after product pressing can also result in the product remaining on the punch due to the vacuum. Improperly cured or tacky surface paperboard coatings can also result in the product sticking to/remaining on the punch half. The various mechanical knock-outs may not be able to remove products with improperly cured/tacky coatings. Corrective action in the coating operation or alternative materials are typically required to improve product removal from the punch half when such coatings are encountered.
A mechanical punch stripper ring knock-out was conceived and developed which operates similarly to the full area punch knock-out but has much less surface area. The stripper ring knock-out still provides clamp and pleating control to the container blank during the initial stages of forming, but provides more positive, consistent product release after forming. Less surface area results in less vacuum force between the stripper ring knock-out and pressed product. The product releases from the punch half more quickly and consistently and is more easily assist air ejected from the forming die set without experiencing the Bernoulli Effect lift. The formed product may even remain in the lower die half when the stripper ring knock-out is used resulting in yet easier air assist product removal.
The stripper ring knock-out in accordance with the invention may have a stationary center piece that limits the stroke distance during actuation. A separate ring portion actuates and separates the formed product from the punch contour. The weight of the formed product more easily overcomes the lesser amount of vacuum generated forces resulting from the lesser stripper ring surface area. The stripper ring knock-out may be actuated by gravity (weight of ring or ring plus shaft), springs, pneumatic or any other suitable means. The stripper ring moving component should have several vent holes through it to prevent creation of a vacuum or billow type effect inside the ring when it actuates. Vent holes may also pass from behind the moving stripper ring component to the inside of the punch cavity to prevent air back pressure on its upward stroke and allow air to more quickly flow in during its eject stroke as can be seen from the appended Figures.
A stripper ring knock-out may be designed for circular, oval, or square style plates, bowls, and trays with the same overall assembly, small contact area concept such as to provide quick, consistent pressware product release. It is possible, that a stripper ring assembly may be designed that provides the same benefits as the articulated full area punch knock-out, but with a lesser perimeter area only. It is also possible that a spoked design may be produced with less area which would also provide a benefit. The invention described herein may be used for matched metal forming of paperboard, paper plastic composites and so forth for disposable food service containers.
In a pressware die set including a punch, or male half and a female, or die half, both halves are typically mounted in an oblique, or angled manner with respect to a vertical plane wherein the punch is most typically upwardly located with respect to the die and reciprocates toward the die. It is possible, however, to locate the die upwardly and have it reciprocate toward the punch or male half depending upon the particular apparatus employed.
There is provided in accordance with the present invention an improved reciprocating pressware die set including a male punch and a female die wherein the punch is a segmented punch for fabricating a pressed disposable food service article having a substantially planar bottom portion, a sidewall portion and a rim portion. The die set is mounted to reciprocate cyclically between a closed position and a forming or open position about an axis of reciprocation. The punch includes an outer punch surface for forming sidewall and rim portions of the pressed disposable food service articles and a central punch member suitably mounted inwardly with respect to the outer punch surface wherein the central punch member and optionally a portion of the outer punch surface defines a substantially planar punch surface configured to engage the planar bottom portion of the article during a forming cycle. The apparatus further includes a knock-out stripper member juxtaposed with the substantially planar surface of the punch. The knock-out surface of the knock-out stripper member is reciprocally mounted with respect to the substantially planar punch surface, such that the knock-out surface is contiguous with the substantially planar punch surface of the punch and forms a part thereof when the punch is in the forming (closed) position, and the knock-out surface of the knock-out stripper member projects axially from the substantially planar punch surface towards the female die when the die set reciprocates toward the open position. The inventive apparatus is further characterized in that the area of the knock-out surface is from about 3 to about 50% of the surface area of the substantially planar punch surface.
The area of the inventive knock-out surface is more typically from about 6 to about 30% of the surface area of the substantially planar punch surface and preferably from about 8 to about 20%. The knock-out surface of the knock-out stripper member may reciprocate over a distance of from about 1 inch to about {fraction (1/16)} inch from the substantially planar surface of the punch, while from about xc2xd inch to about xe2x85x9 inch from the substantially planar surface of the punch is perhaps more typical. In a preferred embodiment, the knock-out stripper member comprises an annular ring concentrically disposed at about the periphery of the central punch member, and may be biased to project downwardly toward a lower die in the open position. The annular ring may have a width (i.e., the distance per side between its inside diameter and outside diameter) of anywhere from about 0.062 inches to about 1 inch with from about 0.12 inches to about 0.5 inches being more typical. In most preferred embodiments, an annular ring may have a width of from about 0.18 to about 0.31 inches. Biasing means include a spring or a weight as further described herein. A gravity biased ring is preferred wherein the ring itself is of sufficient weight or the ring is coupled to a central shaft to provide additional weight.
In another aspect of the invention there is provided a method of making a pressed disposable food service article having a substantially planar bottom portion, a sidewall portion and a rim portion comprising disposing a container blank in a reciprocating pressware die set including a male punch and a female die, the punch being a segmented punch comprising:
(i) an outer punch member for forming the sidewall and rim portions of the pressed disposable food service articles and optionally configured to form part of the bottom portion of the article;
(ii) a central punch member inwardly mounted with respect to the outer punch member wherein said central punch member and optionally said outer punch member define a substantially planar punch surface configured to engage said substantially planar bottom portion of said article during a forming cycle; and
(iii) a knock-out stripper member juxtaposed with the substantially planar surface defining a knock-out surface and being reciprocally mounted with respect to the substantially planar surface of the punch such that the knock-out surface is contiguous with said substantially planar punch surface and forms a part thereof when the punch is in the forming (closed) position and the knock-out surface of the knock-out stripper member projects axially from the substantially planar punch surface towards the female die when the die set reciprocates towards the open position. The area of the knock-out surface is from about 3 to about 50% of the surface area of the substantially planar surface of the punch. The second step of the process is pressing the container blank in the pressware die set. Typically, the process is practiced with paperboard container blanks having a thickness or caliper of from about 0.008 to about 0.050 inches. Thicknesses of from about 0.012 to about 0.025 inches are usually preferred.